In terms of network performance, packet data latency is one of the key metrics for operators, vendors and end users. Latency measurements are performed during all phases of a radio access network (RAN) lifespan. Long Term Evolution (LTE) bad been hailed to have better latency figures than other Radio Access Technologies (RATS) available now. A key differentiator for 5G from LTE would be latency figures.
Reduced latency ha always been a key building block of any new RAT. The benefits of reduced latency are immense, including increased system capacity and enhanced user experience. In some cases, reduced latency may also lead to a faster control plane signaling, implicitly supporting more dynamic control of the network to optimize the available network resources in the best possible manner.
Currently, LTE uplink (UL) ensures that the UL transmissions received from different wireless devices within a cell arrive at the eNodeB (eNB) at the same time, thus maintaining UL timing at the eNB and avoiding inter-subframe interference. If there are small timing misalignments between received signals, the cyclic prefix of the transmission can compensate for it. In LTE the mechanism to control the receive timing is called UL time alignment.
Timing advance is an offset at the wireless device between the start of a received downlink (DL) subframe and a transmitted UL subframe. The network controls the timing of the received signals from a wireless device by controlling this offset.
Timing advance is, ideally, directly proportional to propagation delay. A wireless device far from a base station experiences higher propagation delay, while wireless devices close to the base station experience less propagation delay. Thus, wireless devices at cell edge must start their transmission earlier than wireless devices at the cell center so that both transmissions arrive in-sync at the network receiver.
The network continuously monitors the UL transmission, estimating received signal propagation delay and, if required, instructs the wireless device to adjust its transmission timing using a timing advance command. Besides the normal UL sounding signal, the network can use any UL transmission to estimate the propagation delay.
The time alignment is performed starting from initial attach, then at regular or irregular intervals during the wireless device's stay in a cell (as well as in a target cell after handover). At initial access, the wireless device sends a random access preamble. The network measures the timing offset of the preamble, and sends a timing offset back in the random access response with a timing advance Medium Access Control (MAC) Control Element (CE). The wireless device then adjusts its timing per the timing offset in the random access response before transmitting in the UL.
The network also continuously monitors the UL timing or the wireless device, and determines whether the timing offset is within a targeted range (compared to the ideal timing). The network transmits a Timing Advance Command to the wireless device when the timing offset is outside the targeted range. The network might periodically send Timing Advance Commands to the wireless device even if no change of timing offset has taken place. If the wireless device timing alignment timer expires, the wireless device has to do a random access to restore the timing.
During handover, the wireless device receives a RRC Connection Reconfiguration message from the source network commanding the wireless device to perform the handover. After receiving the RRC Connection Reconfiguration message, the wireless device performs synchronization to a target network and accesses the target cell via a random access preamble on the Random Access Channel (RACH). The target network measures the timing offset of the preamble, and responds with an UL allocation and with a timing advance MAC CE. The wireless device then adjusts its timing per the timing offset value before transmitting RRC Connection Reconfiguration Complete message to indicate that the handover procedure is completed for the terminal.